Definitions of Mach numbers:
\[
\boxed{
\begin{aligned}
M_\infty &\rightarrow \text{Free-stream Mach number}
M_{cr} &\rightarrow \text{Free-stream Mach number at first local sonic point}
\end{aligned}
}
\]
At \(M=M_{cr}\), the local Mach number on the airfoil first becomes
\[
\boxed{1.}
\]